This invention relates to a novel composition for an electric cell comprising a lithium anode and a copper oxide or cadmium oxide cathode with an organic electrolyte potassium hexafluorophosphate-propylene carbonate. The electric cell is designed to supply the electrical energy for a watch module for a period in excess of one year while maintaining a potential sufficiently high to ensure proper circuit operation.
In development of an electrochemical system, certain prerequisites are needed. There must be an anode and a cathode in some form of electrolyte which conducts ions but not electrons. This electrolyte may be a solid, liquid or gas, with liquid being the most common. The electrolyte of an electric cell is a solvent which acts as a transport medium and a solute which actually carries the ions between the anode and cathode. For many electrical cell applications water is the solvent of choice. Where small size and long life are important, water has been found to be unsatisfactory as a solvent choice.
Usually small watch electric cells use a light metal anode, particulary lithium. An organic solvent is required with a lithium anode due to the intense reactivity of lithium with water according to the reaction: Li+H.sub.2 O.fwdarw.LiOH+1/2H.sub.2. A lithium anode electrical cell using water as a solvent would lead to an electric cell with a short shelf life due to loss of the lithium anode and to generation of hydrogen pressure within the cell which would be undesirable in sealed cells which are commonly used in electric watches. Therefore, the use of an organic solvent in a lithium anode electric cell is necessary to achieve a small sealed electric cell with desirable commercial characteristics of long shelf-life and long steady use-life.
Unfortunately, many organic solvents are unstable in the presence of lithium. Therefore, it is necessary to find and utilize a solvent which is unreactive to lithium. One such suitable solvent is known to be propylene carbonate. Another known suitable solvent is nitromethane. A wide variety of other solvents exist which are known to be stable in the presence of lithium. The solvent of the electrolyte must be stable in the presence of the chosen electric cell cathode in addition to demonstrating stability in the presence of a lithium anode.
An electric cell cathode is initially chosen to meet two significant criteria: (1) The electrochemical potential difference between the cathode and the lithium anode, as predicted from thermodynamics, is of sufficient magnitude to be usable in the cell application; (2) The anode and cathode should be chosen to produce the highest energy density possible. Regarding the first criterion, while thermodynamic calculation may predict the electromotive force (emf) of the cathode and lithium anode under certain prescribed circumstances, it may in practice be found that a substantially different reaction results and indeed a different potential exists. A case in point is the use of a lithium anode with a manganese dioxide cathode. If this were to discharge according to the reaction:
4 Li+MnO.sub.2 .fwdarw.Mn+2 Li.sub.2 O PA1 2 Li+2 MnO.sub.2 .fwdarw.Mn.sub.2 O.sub.3 +Li.sub.2 O
a potential of about 1.5 volts would be obtained from the free energies of formation of the compounds used. In practice, however, it is found that a more likely reaction is:
which yields a predicted emf of about 3.0 volts. Thus while a particular electrochemical anode and cathode couple is selected for a particular application by established thermodynamic methods, only empirical studies will show the true reaction and capabilities of the system.
While the theoretical potential, i.e. the electrical potential available from a selected anode-cathode couple, is easily calculated, there is a need to choose a non-aqueous electrolyte that permits the actual potential produced by the complete electric cell to approach the theoretical potential to a practical degree. It is practically impossible to predict in advance how well a non-aqueous electrolyte will function in this respect with a selected couple. More broadly stated, an electric cell must be considered as whole units, each unit having three parts which parts are not predictably interchangeable from unit to unit.
The second criterion for choosing an electric cell cathode is the anode-cathode energy density. The anode-cathode couple with the highest energy density is the most desireable. This usually is determined on either a volumetric or gravimetric basis. For volumetric needs a high density, low atomic or molecular weight material is chosen. For a gravimetric application, a low density, low atomic weight or molecular weight material will be the choice. Additionally, the number of electrons that an element or compound may gain or lose will act as a multiplier of the energy density. While lithium has a low density, its low atomic weight allows it to be a viable anode for volumetric as well as gravimetric applications. The oxides of metals have been found to be particularly suitable cathodes for use in gravimetric applications, with the oxides of heavy metals having suitable densities for volumetric energy density considerations.
Unfortunately, one cannot just select a particular electrochemical anode-cathode couple and one of the known electrolytes and make an electric cell. In addition to the reactive considerations discussed above, it may be found that either the lithium anode or the cathode selected are unreactive in the chosen solvent. Passivating layers may form on the surface of the lithium anode, or the selected cathode may react at such a low rate as to become severely polarized which results in a drop in potential and resulting lack of ability to do useful work. The selected solvent must exhibit adequate conductivity by virtue of its ability to solvate the solute ions. The viscosity of the solvent must be low enough not to impede the transport of the solvated ions. This is all clearly a trial and error process after the original theoretical calculations indicate the possibility of a viable potential. Thus, development of an electrochemical system, while guided by theoretical considerations, requires trial and error methods to determine if the system is viable in the real world.
After an appropriate anode-cathode couple and solvent have been chosen a suitable solute for the electrolyte must be determined. One of the first considerations is that the solute must be stable with the chosen solvent. Of course, the solute must be soluble in the solvent. Also the solute must be capable of carrying ionic charges during discharge from the cathode to the anode and vice versa. Ions must be transported both to the anode (negatively charged ions) and to the cathode (positively charged ions). The ionic components of the solute act to transport electrons from the cathode to the anode which then flow back to the cathode through the external circuit, thereby doing useful work. Therefore a good solute is critical to the successful creation of an electric cell.
Therefore, it is an object of this invention to compose an electric cell which exhibits a potential voltage under normal load conditions which is suitable for watch battery applications.
It is a further object of this invention to compose an electric cell using an organic solvent which exhibits a shelf-life which is far superior to comparable aqueous electric cells, thus giving improved commercial utility and eliminating gas discharge to allow the electric cell to be sealed.
Finally, it is an object of this invention to provide an electrolyte which will function with the anode-cathode couple of lithium and copper oxide or cadmium oxide in a manner to give long use-life, long shelf-life and steady output potential.